1. Field of the Invention
The present invention relates to a needle-less injector that can deliver a high-pressure jet of fluid, such as a medicament, intramuscularly, intradermally and/or subcutaneously into the tissue a human or animal, and more particularly to a method of delivering a specific dose of medicament via a needle-less injector.
2. Description of Related Art
The advantages of needle-less injection devices have been recognized for some time. Some of these advantages include: the absence of needle stick injuries that present hazards to healthcare workers; a reduction in the risk of cross-contamination among patients, whether, human or animal; the elimination of needle breakage in the tissue of the human or animal; and that the jet of liquid medicament is generally smaller than the diameter of a hypodermic needle and thus may be less invasive than a hypodermic needle.
Because of the well-known advantages of needle-less injection, there are many different kinds of such devices, including pneumatic needle-less injection devices that are designed to provide multiple doses to patients or animals, or gas actuated, which are for single or multiple use. Most known needle-less injection devices operate by using a piston to drive the fluid to be delivered though a fine nozzle that creates a small, high pressure stream that penetrates the skin simply due to the high pressure. Multi-dose and single-dose devices depend on a source of energy to drive air or working fluid that is used to operate the piston that drives the fluid through the nozzle. Thus, a serious limitation of these devices is that they must have a readily available source of energy to drive the piston. This makes these devices impractical for use in hospitals and/or clinics, and in most field situations, especially in remote areas where access to dependable energy is uncertain.
These injector devices are also large, sometimes expensive units, and generally adapted to retain large quantities of medicament for repeated injections. Most of these machines are not portable and have historically been used chiefly for mass inoculation programs.
Because of the disadvantages of injection devices that use high-pressure fluids to drive the piston and deliver multiple injections, a great deal of attention has been given to the development of a spring-powered needle-less injection device for delivering a single injection. The success of the known devices has been limited, due to problems associated with safety and reliability. The issues regarding safety generally involve the possibility of accidental discharge of the device and the possibility of transmitting diseases between patients due to carryover of body fluids. The problems associated with reliability generally involve the device's ability to deliver a full, known dose of the liquid.
Safety issues generally arise in association with devices that have exposed triggers or include a hammer or piston driving device that can extend beyond the inner housing of the injector. The risk of using this type of device is similar to the risks associated with the triggers on firearms, and that is the inadvertent pressing of the trigger, can result in the accidental or premature firing of the device.
Reliability issues include a broad spectrum of problems. One significant problem is the creation of a suitable jet or stream of fluid and the introduction of this jet on to the skin of the animal or human. Preferably, the jet will be a very fine jet that will impact a section of taut skin at an angle of incidence of preferably 90 degrees. Most of the energy of the stream is used to penetrate the skin when the jet impacts at approximately 90 degrees to the skin. Additionally, by keeping the skin taut prior to delivering the jet of fluid, the skin is not allowed to flex, and thus more of the energy from the jet is used to penetrate the skin rather than deflecting or moving the skin.
Yet another problem associated with needle-less devices is maintenance of a required amount of pressure during the delivery of the medicament from the reservoir, through the nozzle. As disclosed in U.S. Pat. No. 6,942,638, the entirety of which is hereby incorporated by reference, a loss of pressure can affect the amount of medicament delivered.
There are also disadvantages related to the containment of the fluid formulations in single dose needle-less injectors. Individual doses of a liquid formulation can be delivered via the injector. However, often the volume of medicament held in the conventional injectors is too large, for example, when injecting an infant or small animal, such as a mouse. Often one-half or more of the dosage is not required and hence would be wasted or the injection could not be given safely to such patient. This decreases the practicality and use of the injectors in certain environments.
Another problem with medicament containment is that many materials proposed for the vials are unsuitable for long-term contact with the medicament, or at least would require extensive and costly validation for each application.
Another disadvantage of known needle-less injectors is the inability to direct the location of the injection, i.e., intramuscularly, intradermally and/or subcutaneously